1. Field of the Invention
This invention relates to an optical waveguide for use at long wavelengths and more particularly, to single mode low-loss optical waveguides which are especially suitable for the propagation of light, the wavelength of which is in the range between 1.1 and 1.8 .mu.m.
Since the time that optical fiber waveguides were first considered for use in optical communication systems, attempts have been made to develop fibers having lower loss and higher information capacity. It has been known that scattering loss decreases with wavelength and that the scattering loss component can be less than 1 dB/km at wavelengths greater than 1 .mu.m. There is current interest therefore in the development of fibers for use in the range between 1.1 and 1.8 .mu.m. Some materials such as P.sub.2 O.sub.5 and B.sub.2 O.sub.3, which are suitable for use at shorter wavelengths, have been given less consideration for use in the infrared region because of the infrared absorption loss characteristic of such materials. However, oxides such as GeO.sub.2, Sb.sub.2 O.sub.3 exhibit suitably low loss in the infrared region when employed as a dopant for silica.
There is current interest in the use of single-mode fibers for use in the infrared region since, in addition to the possibility of achieving low loss, such fibers enable very high capacity telecommunication systems to be designed with fewer repeater stations. Single-mode fibers having losses less than 1 dB/km have been reported. These fibers, which contain cores of silica doped with germania, were prepared by a chemical vapor deposition technique.
The optical characteristics of a single mode fiber are determined mainly by the parameter V which is expressed by the formula ##EQU1## where a is the core radius, .lambda. is the wavelength of the propagated light and n.sub.1 and n.sub.2 are the refractive indices of the core and cladding, respectively. In the range of the value of V no larger than about 2.4, single mode propagation will occur for step index fibers.
2. Description of the Prior Art
Single mode waveguides having losses less than 1 dB/km in the infrared region have been made by a chemical vapor deposition process whereby reactant vapors flowing through a glass tube react to form particles which are deposited on the inner surface of the tube where they are consolidated to form a glassy layer. One or more layers of glass are formed on the inner surface of the tube in this manner. Ordinarily, the coated bait tube has at least two compositional regions. The interior region, which ultimately forms the core of the resultant optical fiber, consists of SiO.sub.2 and GeO.sub.2. The exterior region, which forms the cladding, consists essentially of SiO.sub.2. The remaining critical step involves pulling the relatively large diameter cylindrical preform into a relatively small diameter fiber. Prior to pulling the preform into a fiber, the preform is usually collapsed into a smaller diameter preform or preferably into a solid cylindrical mass.
Fabrication of this type of fiber is difficult for several reasons. The higher deposition temperature required to deposit pure SiO.sub.2 leads to tube shrinkage during deposition and necessitates the use of pressure control of tube diameter. Deposition rates are reduced because of the difficulty of sintering the particles of SiO.sub.2 that are deposited to form the cladding. The tube collapse step is very difficult because of the high viscosity of the deposited glass. The high temperature required to collapse the coated bait tube leads to a larger index dip at the fiber axis and causes a greater diffusion of hydroxyl ions from the substrate tube toward the core material. Also, because of these processing difficulties, only relatively small preforms have been formed by this technique.
Dopants have been added to the deposited cladding layer to lower the deposition and sintering temperature of the process. The addition of a small amount of P.sub.2 O.sub.5 to the deposited silica cladding layer is taught in the publication: S. Sentsui et al., "Low Loss Monomode Fibers with P.sub.2 O.sub.5 --SiO.sub.2 Cladding in the Wavelength Region 1.2-1.6 .mu.m", Fifth European Conference on Optical Communication, Amsterdam, September, 1979. The use of P.sub.2 O.sub.5 along with either B.sub.2 O.sub.3 or F in the deposited silica cladding layer is taught in the publication: B. J. Ainslie et al., "Preparation of Long Length of Ultra-Low-Loss Single-Mode Fiber" Electronics Letters, July 5, 1979, Vol. 15, No. 14, pp. 411-413. The use of such dopants has resulted in a deposition temperature of about 1500.degree. C., which is approximately 200.degree. C. lower than the temperature required to deposit the pure silica cladding layer. The reduced deposition temperature resulting from a P.sub.2 O.sub.5 level of 0.3 to 1.0 wt.% increases the deposition rate, greatly reduces tube shrinkage and distortion, and makes the collapse process much easier.
During the manufacture of single-mode waveguides by the aforementioned chemical vapor deposition process residual water may be introduced into the light region of the fiber by several mechanisms. By residual water in glass is meant that the glass contains a high level of OH, H.sub.2 and H.sub.2 O. Water can be introduced by employing a bait tube containing a large amount of water, by employing reactants which contain water, or it can enter through leaks in the vapor transport system.
The main disadvantage of using P.sub.2 O.sub.5 in the cladding is that OH radicals trapped at phosphorus cites result in a broad P--O--H absorption band which increases attenuation in the 1.3-1.8 .mu.m range. A discussion of P--O--H absorption in the infrared region of the spectrum appears in the publication: T. Edahiro et al. "Spectral Loss Characteristics of GeO.sub.2 --P.sub.2 O.sub.5 Doped Silica Graded-Index Fibres in Long-Wavelength Band", Electronics Letters, May 10, 1979, Vol. 15, No. 10, pp. 274-275.